<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        if (!undefined) {
            console.log('undefied is false')
        }
        if (!null) {
            console.log('null is false');
        }
        // null is false

        console.log(undefined == null) // true
        console.log(undefined === null) // false
        console.log(Number(null)) // 0
        console.log(5 + null) // 5
        let i;
        console.log(i); // undefined

        function fun(x) {
            return x
        }
        console.log(fun()) //undefined
        function fun1(){}
        console.log(fun1()) // undefined
    </script>
</body>

</html>